1. Technical Field
Embodiments generally relate to electronic circuits and, more specifically, to radio frequency couplers. Embodiments more specifically relate to a dual coupler.
2. Discussion of the Related Art
A coupler is generally used to divert part of the power present on a so-called main or primary transmission line, towards another so-called coupled or secondary line, located close to it.
Couplers can be divided into two categories according to whether they are formed of discrete passive components (lumped-element coupler) or of conductive lines which are close to each other to be coupled (distributed coupler). Embodiments relate to the second category of couplers.
In many applications, it is needed to sample part of the power transmitted over a line, for example, to control the power of an amplifier in a transmit circuit, to control the linearity of a transmit amplifier according to the loss due to the reflection of an antenna, to dynamically match an antenna, etc. A coupler is used to sample this information.
A dual coupler shares measurement ports between two transmission lines intended to convey signals in two different frequency bands. Such a sharing is possible in any dual system where the frequency bands are not used simultaneously. Such is generally the case for radio applications (for example, mobile telephony for a dual-, tri-, or quad-band phone, Wi-Fi, etc.). A dual coupler for example enables sharing the same control or amplification circuit for two transmission paths.
However, in a dual coupler, the antennas connected at the output of the two main lines introduce an additional coupling. The greater this coupling (the poorer the isolation between the two antennas), the more the measurement results are distorted. The coupler is then not sufficiently frequency-selective for one path over the other.
US-A-2005/0239421 discloses a directional dual coupler with capacitive compensation. The signal of the secondary lines is drawn through a diplexer. The other ends of the secondary lines are grounded by resistors.
It would be desirable to improve the selectivity of a dual coupler.
It would also be desirable to have a symmetrical arrangement.